The BR Class 9F was born out of the need for British Railways requirement for a heavy freight locomotive and was part of their standardisation plan. By 1954 the first 20 Class 9F locomotives had been introduced, however with coal economies in mind it was decided as an experiment that the next 10 of the Class to be built should be built using Franco-Crosti boilers.
Named after two Italian engineers who had worked for the Italian State railway in the 1930s, the Franco-Crosti boiler was a modification of the more conventional fire tube boiler, which was used on the majority of steam locomotives. Different from the more common type of boiler, the heat that remained in the exhaust gasses was used to preheat the water supply for the main boiler by using a secondary heat exchange system. The heat exchange system was in effect a secondary boiler. From there the preheated water was fed at full boiler pressure into the main boiler. The feedwater heater as it was known was not designed to produce steam but to raise the temperature of the water allowing the heater to maximise the remaining energy in the exhaust gasses more efficiently.
The Franco-Crosti system had proved quite successful in Italy and Germany and so in 1955 the 10 experimental 2-10-0 locomotives which had slightly smaller boilers than the standard 9Fs were fitted with a single feedwater heater located underneath the main boiler. The familiar 9D funnel was retained, however it was only used for “firing up” after which it was blanked off when the locomotive was operating. The actual Crosti chimney was located on the right side of the locomotive slightly forward of the firebox.
Unfortunately the Crosti design did not show a particularly marked improvement in operation and proved to be a concern in respect of maintenance due to the acidic flue gasses condensing in the feed water heater with the inevitable result of corrosion. After further test using the standard and more conventional 9Fs as a comparison, the small improvements in efficiency when compared against the additional costs of fitting the system, plus payments to Crosti and the fact that the locomotives were slightly less powerful, the decision was made to remove the system and convert the locomotives to a standard type boiler. However, even after conversion the ten affected locomotives still retained their smaller boilers.